An angular multiplexing apparatus for a holographic storage medium is, traditionally, classified into one using a plane reference beam as shown in FIG. 1A and the other using a spherical reference beam as shown in FIG. 1B according to a reconstructing reference beam used therein.
FIG. 1A shows a holographic Rom system including a conventional angular multiplexing apparatus for a holographic storage medium using the plane reference beam. The plane reference beam provided from a laser 1 propagates along a path S1; is reflected sequentially by a first and a second mirror 2, 4; is transmitted sequentially through a first and a second lens 5, 6; and is projected onto a reconstructing position P of a holographic storage medium 8. Subsequently, the plane reference beam is diffracted by a holographic interference pattern in the holographic storage medium 8 to generate a reconstructed beam, which is detected by a detector 9.
The first and the second lens 5, 6 serve to substantialize angular multiplexing in the holographic storage medium 8 using the plane reference beam. Specifically, when focal lengths f of the first and the second lens 5, 6 are equal to each other, the second mirror 4, the first and the second lens 5, 6 and the holographic storage medium 8 are disposed such that a distance from the second mirror 4 to the first lens 5, a distance from the first lens 5 to the second lens 6, and a distance from the second lens 6 to the reconstructing position P of the holographic storage medium 8 are f, 2f, and f, respectively. As shown with dashed lines in FIG. 1A, it is possible to implement the angular multiplexing in the holographic storage medium 8 since a rotation of the second mirror 4 causes an angle of reflection of the plane reference beam on the second mirror 4 to be varied so that an incidence angle of the plane reference beam onto the holographic storage medium 8 after being transmitted through the first and the second lens 5, 6 is varied.
However, the angular multiplexing apparatus shown in FIG. 1A is only for the plane reference beam and, therefore, it has been difficult to form a focus at the reconstructing position P of the holographic storage medium 8 when a spherical reference beam is used instead of the plane reference beam.
FIG. 1B illustrates a holographic Rom system including another conventional angular multiplexing apparatus for the holographic storage medium using the spherical reference beam. After a plane reference beam provided from a laser 11 is reflected by a first mirror 12, the plane reference beam is transmitted through a lens 13-1 to be converted into the spherical reference beam. The spherical reference beam propagates along a path S2; is reflected by a second mirror 14-1; and is projected onto a reconstructing position P of a holographic storage medium 18. Subsequently, the spherical reference beam is diffracted by a holographic interference pattern in the holographic storage medium 18 to generate a reconstructed beam, which is detected by a detector 19.
The lens 13-1 and the second mirror 14-1 serve to substantialize angular multiplexing in the holographic storage medium 18 using the spherical reference beam. Specifically, in order to change an incidence angle of the spherical reference beam onto the reconstructing position P of the holographic storage medium 18, the lens 13-1 must be moved along the path S2 of the spherical reference beam and the second mirror 14-1 must be rotated to have a predetermined inclination angle as well as be moved along the propagation path S2 of the spherical reference beam. In other words, in order to change the incidence angle of the spherical reference beam onto the holographic storage medium 18, as shown with dashed lines in FIG. 1B, the mirror 14-1 must be changed into a mirror 14-2 or 14-3 with the inclination angle thereof to be rotated and the location thereof to be moved. Further, in order to form a focus of the spherical reference beam precisely at the reconstructing position P of the holographic storage medium 18, as shown with dashed lines in FIG. 1B, the lens 13-1 must be moved to form a lens 13-2 or 13-3 such that a path distance from the lens 13-1, 13-2 or 13-3, i.e., a generating position of the spherical reference beam, to the reconstructing position P of the holographic storage medium 18 is equal to a focal length of the spherical reference beam. Therefore, three degrees of freedom such as a translation of the lens 13-1, a translation and a rotation of the mirror 14-1 must be controlled connectively to substantialize the angular multiplexing when the spherical reference beam is used.